1. Field of the invention
The present invention relates to a copying machine control apparatus, and more particularly to a improved copying machine control apparatus which can eliminate unwanted elongation, shortening, or misalignment of the copied product.
2. Prior Art
The conventional color copying machine generally comprises a light source, movable mirrors, a photoconductive drum, and a transfer drum. These components are accurately driven in accordance with a predetermined timing to form an original on a record sheet.
FIG. 1 is a diagrammatic illustration showing the construction of a conventional color copying machine. The color copying machine has frame 1, the upper portion of which is provided with platen 2 for placing an original thereon. Scanning unit 3 is arranged below platen 2, and it includes elongated lamp 4, first and second mirrors 5 and 6, a filter-and-lens unit 7, and third and fourth mirrors 8 and 9. Lamp 4 is mounted on frame 1 so as to move in a linear to and from direction with first mirror 5, as indicated by arrows A and B in FIG. 1. Second mirror 6 is arranged so as to move in accordance with movement of lamp 4 and first mirror 5 at a speed which is half the speed of the movement of lamp 4 and first mirror 5.
During operation, lamp 4 and first mirror 5 are moved in the direction indicated by arrow A. As a result, an outer peripheral surface of photoconductive drum 10 is exposed to a beam of light which forms an image on platen 2. In this case, filter-and-lens unit 7 has been changed over so that it passes on components of the light other than the yellow component. Photoconductive drum 10 has also been electrically charged by charging device 11. Therefore, an electrostatic latent image corresponding to the yellow component of the image on the original is formed on the peripheral surface of photoconductive drum 10. A yellow toner is then allowed to adhere to the electrostatic latent image on photoconductive drum 10 by first developing device 12, so that a yellow toner image is formed on the peripheral surface of photoconductive drum 10.
A record sheet is fed from record-sheet trays 13 or 14 and is wrapped around transfer drum 15, and fed to a transfer position T where the record sheet is fastened by gripper 16. By transferring the record sheet to copying position T, the front edge of the record sheet is aligned with that of the yellow toner image. Then, the image begins transferring the yellow toner image on photoconductive drum 10 is transferred to the record sheet on transfer drum 15. At this time, the peripheral velocity of photoconductive drum 10 is equal to that of transfer drum 15. The peripheral surface of photoconductive drum 10 is in turn cleaned by cleaning device 17 while the copy is being finished.
Accordingly, when the transfer of the entire yellow toner image to the record sheet is completed, filter-and-lens unit 7 is changed over so that it passes color components of the light other than the magenta component. At the same time, second developing device 18 for the magenta color is selected, and another transfer operation is carried out in the manner described for the yellow toner image. Then, filter-and-lens unit 7 is again changed over so that it passes color components of the light other than the cyan component, and third developing device 19 is selected. Further transfer operations are carried out in the same manner. Thus, the toner images of the three primary colors yellow, magenta and cyan are applied to the surface of the record sheet, which is wrapped around transfer drum 15, to form color images.
The record sheet on transfer drum 15 is then fed by endless belt 20 to fixing device 21 at which the color image formed on the record sheet is fixed thereto. When the fixin operation is completed, the record sheet is discharged to tray 22, terminating the copying operation.
FIG. 2 shows the brief drive layout of a color copying machine control apparatus. In this drawing, numeral 31 designates an electric motor for driving photoconductive drum 10, while numeral 32 designates another electric motor for driving transfer drum 15. The rotations of respective motor shafts 31a and 32a rotate respective shafts 10a and 15a through couplings 33 and 34. Shafts 31a and 32a are extended from photoconductive drum 10 and transfer drum 15, respectively. Electric motor 31 has pulse encoder 35, while electric motor 32 has pulse encoder 36. Both pulse encoders 35 and 36 generate pulse trains Pl and P3 in response to the rotation of respective electric motors 31 and 32 to output to respective down-count terminals D of counters 37a and 38a. These counters 37a and 38a are incorporated in controllers 37 and 38, respectively. Controllers 37 and 38 are described later.
Numeral 39 designates an electric motor for moving scanning unit 3, in which motor shaft 39a rotates pulley shaft 41 to move elongated lamp 4 and the like in a linear to and fro movement. Electric motor 39 has pulse encoder 42 to generate pulse train P2 in response to the rotation thereof and output pulse train P2 to down-count terminal D of counter 43a which is incorporated in controller 43.
Controller 37 controls the rotation of electric motor 31 so that the rotation speed of photoconductive drum 10 is equal to the rotation speed which is indicated by reference pulse train CM1 outputted from reference signal generator 44. That is, reference signal generator 44 supplies reference pulse train CM1 having a predetermined frequency to up-count terminal U of counter 37a which is incorporated in controller 37. In addition, controller 37 controls the rotation of electric motor 31 so that the frequency of pulse train Pl outputted from pulse encoder 35 to down-count terminal D, is equal to the frequency of reference pulse train CM1. Thus, the rotation speed of electric motor 31 is made equal to the rotation speed which is indicated by reference pulse train CM1.
Controller 38 controls the rotation speed of transfer drum 15 based on the frequency difference between reference pulse train CM3 supplied to up-count terminal U of counter 38a and pulse train P3 outputted from pulse encoder 36 while controller 43 controls the movement speed of scanning unit 3 based on the frequency difference between reference pulse train CM2 supplied to up-count terminal U of counter 43a and pulse train P2 outputted from pulse encoder 42. Thus, pulse train Pl outputted from pulse encoder 35 is used as reference pulse trains CM3 and CM2 to synchronize the rotation of photoconductive drum 10 with the rotation of transfer 15 and the movement of scanning unit 3.
The load of photoconductive drum 10 varies with its condition when the outer peripheral surface contacts one of the first developing device 12, second developing device 18, or third developing device 19, or when it is separated from them. This causes the peripheral speed of photoconductive drum 10 to vary transiently during rotation. In the case where the movement speed of scanning unit 3 is constant, when the peripheral speed of photoconductive drum 10 is relatively lower than the movement speed of scanning unit 3, the toner image formed o photoconductive drum 10 is elongated. Conversely, the image is shortened when the peripheral speed of photoconductive drum 10 is higher than that of scanning unit 3. Furthermore, out of synchronization peripheral speed between transfer drum 15 and photoconductive drum 10 causes misalignment of colors.
In order to prevent the elongation or shortening of the toner image, and misalignment of colors, as described above, pulse train P1 is commonly used as reference pulse trains CM3 and CM2. That is, in controller 38, the use of pulse train Pl synchronizes the rotation of transfer drum 15 with the rotation of photoconductive drum 10 so that the rotation speed of electric motor 31 is followed by that of electric motor 32. In controller 43, pulse train Pl is used as reference pulse train CM2 to synchronize the movement speed of scanning unit 3 with the rotation speed of photoconductive drum 10. At this time, scanning unit 3 in copying mode, moves in the direction of arrow A shown in FIG. 2, when changing switch 45 to position "F" by rotating direction signal F/R. On the other hand, when changing switch 45 to position "R" by rotating direction signal F/R, pulse train P4 as reference pulse train CM2 is supplied to controller 43 from reference signal generator 46 to rapidly return scanning unit 3 to the starting position because the frequency of pulse train P4 is higher than that of reference pulse train CM1.
However, pulse train P1 (as reference pulse trains CM3 and CM2) controls the rotation of electric motors 32 and 39, in which electric motor 32 rotates transfer drum 15 and electric motor 39 drives scanning unit 3 in a linear to and fro movement, so that controllers 38 and 43 respond to electric motor 31 through pulse encoder 35 with a time delay, thus the operation of transfer drum 15 and scanning unit 3 has a time delay in response to the speed variation of photoconductive drum 10.
Photoconductive drum 10 is usually made of aluminum or other such light materials, so that the inertia is relatively small, thus speed variation is large when contacting or disengaging photoconductive drum 10 and one of the developing devices 12, 18 and 19.
According to the above two paragraphs, problems remain such as the elongation or shortening of the toner image, and misalignment of color on photoconductive drum 10.
These problems occur not only in the above copying machine, but also in a copying machine which comprises a single developing device without a transfer drum which is a so-called monochromic-type copying machine.
Furthermore, in the case of a conventional color copying machine, the coupling portion between electric motor 32 and transfer drum 15 or the linkage comprises coupling 34, shaft 32a, and shaft 15a, and has a relatively low torsional rigidity, so that a torsional vibration is produced when increasing or decreasing the rotation speed of transfer drum 15. Thus, in FIG. 3, the rotation speed of transfer drum 15 changes in response to torsional vibration during copying mode, and therefore the front edge of the record sheet is not aligned with the front edge of the toner image.